Ampeg B3 (3158) problem

[MarkusBass]

Let me quote a document from "Critical thinking" thread by Enzo in Music Electronics forum:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Don't give up - review the situation. Chances are the problem may have been overlooked. Don't be afraid to start over again. Try different checks, look at different aspects of the
problem and check out things that may not have been considered important the first time around. Widen the scope of the investigation. It is always well to remember:
"There is a logical reason for everything that happens."
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
I don't know why removing R7 helps but I think it's something else. I told you to test the voltage follower (did you do it?). If you can put let's say 20V(p-p) signal into the RETURN jack (with disconnected power amp) and get undistored signal on TP2, there is nothing wrong with this part of the circuit. Have you tested other parts of the amp separately? Have you checked the power amp? The reason I'm asking about it is that I see this limiter in the power amp. And if there is something wrong with e.g. quiescent current of the power amp, or with the LDR element, the amp may work as you describe. So my advise is to check each part of the amp separately. Check the power amp; can you get expected signal on the output? If you need, the limiter can be temporarily disconnected. Then test the Effect Return part of the circuit (disconnect the power amp if you want). Test the preamp. The TL074 ICs in this amp have unused parts - you can use them and build this voltage follower separately from IC1. It clear that something is overlooked - it's only difficult to find. But when you find it, you knowledge will be priceless .

Mark

[Steve Conner]

I'm puzzled by this problem.

According to the op-amp textbook, the circuit might work without R7, but it's not guaranteed to. The inputs leak a small DC current, like the grid of a tube, but much smaller in the case of the TL07x. If this doesn't have a path to ground, it can cause the DC voltage to drift until the op-amp saturates.

It might seem to work when cold, but for JFET op-amps like the TL07x, the input bias current increases drastically when the chip gets hot.

You said there are hardly any decoupling capacitors. Maybe Ampeg didn't use enough? Try adding one (a 0.1uF ceramic, or a 10uF electrolytic, or maybe both) between the power pins of the op-amp that's causing the trouble.

[52 Bill]

Early on (post 13) you stated that there was 40mV dc at TP2. You have since changed a number of parts. Is there still a dc offset at TP2? What happens to this voltage when you remove R7?

[spy]

MarkusBass, with 20Vp-p signal into the return jack I have clean sine & square wave at TP2 but at higher levels say 25-30Vp-p I can see a tooth created at the negative side of the waveform, especially with a square waveform it's larger. The power amp is just fine ( I believe so) because the problem appears before the master volume. Don't forget that this Ampeg has two separately power amps, a low and a high freq with independent outputs and the problem is audible at both outputs.

Steve Conner, thanks for the explanation about R7. So I must leave it on the circuit! I'll try the decoupling caps on the power rails of IC1.
edit: I tried with two 1u decoupling caps and the problem is still there.

52 Bill, I was reffering to IC1 on the preamp board. The IC we are talking now is at the power amp board. Sorry for the confusion. At this IC I have 2mVdc at the output (TP2) and without R7 it's not stable! It rises all the time, I closed the amp when it was up to 500mV!

[MarkusBass]

MarkusBass, with 20Vp-p signal into the return jack I have clean sine & square wave at TP2 but at higher levels say 25-30Vp-p I can see a tooth created at the negative side of the waveform, especially with a square waveform it's larger. The power amp is just fine ( I believe so) because the problem appears before the master volume. Don't forget that this Ampeg has two separately power amps, a low and a high freq with independent outputs and the problem is audible at both outputs.

Now I'm confused. The power amp schematic shows only one power amp, and the preamp schematic does not show any hi-low filter. Is it a correct schematic?
IC1 is supplied with +/-16V. TL074 is not a rail-to-rail opamp so you cannot expect that 30Vp-p signal will be passed as a clean signal.
Since you mentioned the Drive pot I looked at the schematic and found out that the "Drive" part of the circuit is build in the way that unsymmetrical clipping is applied to the signal (D11, D12, D14 diodes). The more drive you want, the more unsymmetrical clipping you get. And this is just by design. I start thinking that there is nothing wrong with the amp. Alternatively, you may focus on the drive circuit and check whether it's working correctly.

Mark

[spy]

I'm sorry but the previous schematics are for the same amp but with one output. I can't find the exact schematics of B3158 but the preamp is exactly the same until the master volume.

About the drive circuit, I have replaced R18, R19, R17, D11, D12, D14, C8, C9, C10 and IC1!

Also, the problem is still there if I use a bass with an external preamp connected to Return input.

Edit:
Just out of curiousity, I bypassed the voltage follower and the whole fx loop and there are no any loud crackles like before but I still can hear on the notes the same weird sounds but they are too low and at the background. But I'm sure what I hear are the cause of the problem, I don't know why they are much louder when the voltage follower is connected.

[MarkusBass]

Also, the problem is still there if I use a bass with an external preamp connected to Return input.

This is interesting - it rules out the "Drive" circiut. But I wonder why haven't you gone one step further - plug signal from external preamp directly into the power amp? This would show whether the problem is with the voltage follower, or maybe with the power amp. Can you check this?
BTW, I assume that yo ruled out the opto and the Peak LED circuit?

Mark

[spy]

The problem is before the master volume that's why I ruled out the output stages.

[Jazz P Bass]

Stupid question?
Did you run a jumper from the FX in to the FX out jacks?

[Enzo]

Owner manual shows diferent amp output setups, including B3158.

http://www.ampeg.com/manuals/b-3.pdf

[MarkusBass]

The problem is before the master volume that's why I ruled out the output stages.

I would still check all the stages (including power supply). It's clear that something very simple is overlooked. Last time I had an amp (and guess what - it was Ampeg) with similar problems, I found out that some crazy guy had disconnected ground from the transformer. The amp worked great with low/medium power but not at high power.

Mark

[spy]

Jazz P Bass, that would be perfect! Hehe! No, I bypassed the whole FX loop circuit.

Enzo, the preamp stage is the same until the master volume. After the master there is a balance/blend pot that splits the signal. Even the output stages are the same but in double, same opto, opamp driver, etc

MarkusBass, I'll try to check everything for one more time!

[JamN]

My Ampeg B3 with all setting off, when powered on, speakers will suck back with a loud low feedback. any suggestions welcomed

[Jazz P Bass]

Welcome to the forum JamN.

If the speaker sticks in one direction, that 'usually' will indicate an output section problem.

[J M Fahey]

Start by disconnecting that speaker NOW .
Then get a multimeter and check whether you have a DC voltage at the amp speaker output terminals.

Anything above 100mV is to be worried about.
You'll probablu have between 10 and 40 volts.

Post results.